linux-kernel - Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel expects so

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Message-ID: <0149c37d-7065-4c72-ab56-4cea1a6c15d0@intel.com>
Date: Tue, 3 Feb 2026 13:08:39 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Shashank Balaji <shashank.mahadasyam@...y.com>, Thomas Gleixner
	<tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
	<bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Suresh Siddha <suresh.b.siddha@...el.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang
	<haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
	<decui@...rosoft.com>, Long Li <longli@...rosoft.com>, Ajay Kaher
	<ajay.kaher@...adcom.com>, Alexey Makhalov <alexey.makhalov@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Jan Kiszka <jan.kiszka@...mens.com>, Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>, Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC: Ingo Molnar <mingo@...e.hu>, <linux-kernel@...r.kernel.org>,
	<linux-hyperv@...r.kernel.org>, <virtualization@...ts.linux.dev>,
	<jailhouse-dev@...glegroups.com>, <kvm@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>, Rahul Bukte <rahul.bukte@...y.com>, "Daniel
 Palmer" <daniel.palmer@...y.com>, Tim Bird <tim.bird@...y.com>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel
 expects so

> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index d93f87f29d03..cc64d61f82cf 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -2456,6 +2456,12 @@ static void lapic_resume(void *data)
>  	if (x2apic_mode) {
>  		__x2apic_enable();
>  	} else {
> +		/*
> +		 * x2apic may have been re-enabled by the
> +		 * firmware on resuming from s2ram
> +		 */
> +		__x2apic_disable();
> +

We should likely only disable x2apic on platforms that support it and
need the disabling. How about?

...
} else {
	/*
	 *
	 */
	if (x2apic_enabled())
		__x2apic_disable();

I considered if an error message should be printed along with this. But,
I am not sure if it can really be called a firmware issue. It's probably
just that newer CPUs might have started defaulting to x2apic on.

Can you specify what platform you are encountering this?